the elbows could have check valves in them. who knows. this is coming from a guy that builds fish ponds. I know from eperience every time I build a system, like I'm doing today actually, I always try to improve every system. I think with the plumbing involved with this PBR, the question was asked,"What do we do if we have to shut down and fix one tube?" Well I'm sure they found a solution and designed the elbows for that. remember they are patent pending.
Their smart people. I'm sure everything is okay.